Originally Posted by jur
Is climbing different due to the resisting force coming through the pedals? Eg, if you're riding a fixie slowly up a hill, if you don't keep on going every second, your going to roll to a stop and then backwards - there is a reversing force from the pedals. While riding flats, there is resistance but it is not driving backwards, it is merely slowing down.
Is that it?
That plus the angle that your body is in going up a mountain. I (unknowningly) use my abs a lot more when climbing.